Volcanic eruptions are known to cause changes in atmospheric temperature. Eruptions eject large amounts of fine ash and sulfur dioxide, which transform into sulfate aerosols—extremely small airborne particles—into the atmosphere. Those tiny sulfate particles reflect solar radiation back into space, preventing the radiation from heating Earth’s atmosphere below.

Most climate models incorporate this aerosol effect into the calculations used to predict future climate trends. Although sulfates are known to linger in the atmosphere for months or years, researchers previously assumed that ash particles fall from the sky soon after an eruption and therefore don’t contribute to cooling.

However, recent discrepancies in climate model predictions and actual global temperatures have led scientists to wonder whether, perhaps, ash plays a larger role in cooling the atmosphere. Here Vernier et al. investigated whether fine ash particles could be reflecting solar radiation long after an eruption.

The authors studied the ash in the atmosphere following the Mount Kelud eruption in Java, Indonesia, on 14 February 2014. On that day, Kelud erupted for a few hours, spewing volcanic material into the stratosphere.

Following the eruption, the researchers tracked the Kelud plume using instruments aboard the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) satellite that measure backscatter, which hints at the shape and size of the particles within the plume and allowed the team to distinguish between ash and sulfate particles. Three months after the eruption, the researchers also launched the Kelud Ash (KlAsh) balloon field campaign from Darwin, Australia, to measure the reflectivity and size of ash and sulfate particles that remained in the Kelud plume. The team released five small sondes, or balloon instruments, and one large one over the course of 10 days.

They discovered that ash particles persisted within the plume months after the eruption, accounting for 20–25% of the plume’s aerosol optical depth, a term that describes the amount of sunlight atmospheric particles can block. The satellite observations also revealed that the ash particles were more prevalent in the lower layer of the volcanic cloud, while sulfate aerosols were generally observed at higher altitudes.

The scientists suggest that fine ash particles from tropical eruptions can remain suspended for longer periods because there is more atmospheric circulation keeping them afloat. The ash from this and other eruptions over the past decade have likely contributed to regulating the global climate, which might partially explain why climate models did not accurately predict current climate temperatures. The findings could help improve the accuracy of climate predictions in the future. (Journal of Geophysical Research: Atmospheres, doi:10.1002/2016JD025344, 2016)

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